An image displaying apparatus using a hold-type display apparatus such as a liquid crystal display apparatus has a problem of degradation of moving picture quality (indistinct edge).
The following explains the degradation of moving picture quality (indistinct edge) in a conventional hold-type display apparatus with reference to FIG. 29. FIG. 29 shows a case where a region of an image signal 75% in luminance level moves in the horizontal direction on a background of a image signal 25% in luminance level.
FIG. 30 shows luminance level distribution for pixels on a 1-horizontal line on a picture, with respect to an input image signal supplied to a frame for such image display.
FIG. 31 shows time transition in a display luminance distribution of a conventional hold-type display apparatus in such a movement of an image in the horizontal direction. An observer gazing a picture generally follows an object moving in the horizontal direction, and therefore, he/she recognizes a luminance level as an integral amount of display luminance level in the direction denoted by an arrow. FIG. 32 is a numeric value of a luminance level for each pixel of an input image signal on a 1 horizontal line in 1 frame period divided into 8 parts. In this figure, the luminance response time of a display apparatus is not taken into account for ease of explanation.
The visible luminance distribution for an observer is an integral value of luminance level which is a mean value of the luminance levels of respective times in the arrow direction, provided that the movement speed of the object (75% luminance region) is 8 pixel/frame. FIG. 33 is a graph showing the distribution of the luminance level. This luminance level is luminance level distribution connected by an inclined line component in the vicinity of a boundary between a region 25% in luminance level and a 75% luminance region. The width of the line component in the horizontal direction is seen as an indistinct edge. This is a reason of the decrease in moving picture quality in a hold-type display apparatus.
Providing a minimum luminance level (black) display period in a part of the display 1 frame period is the easiest way of reducing the indistinct edge. However, in this case, the light state and the dark state are repeated in each frame period in the entire image, and flicker occurs. Further, since the minimum luminance level display period always exists in 1 frame period even when the input image signal is maximum, the luminance level decreases.
The following explains a case where the width of the 75% luminance region is smaller than the transition amount of 1 frame period in the background 25% in luminance level, as shown in FIG. 34.
FIG. 35 is luminance level distribution for pixels on a 1-horizontal line on a picture, with respect to an input image signal supplied to a frame on the image display shown in FIG. 34. FIG. 36 shows time transition of a display luminance distribution in a conventional hold-type display apparatus in such an image movement in the horizontal direction. FIG. 37 is a table of numerical values of the pixels on 1 horizontal line in 1 frame period divided into 8 parts.
The visible luminance distribution for an observer is an integral value of luminance level which is a mean value of the luminance levels of respective times in the arrow direction, provided that the movement speed of the object (75% luminance region) is 8 pixel/frame. FIG. 38 is a graph showing the distribution of the luminance level.
As shown in FIG. 38, this case does not have a large indistinct edge as with the one of FIG. 33; however, the luminance level of the object which is supposed to move with a luminance level of 75% is decreased to 44%.
This means that the moving object is seen a lot darker than it should be. This is another reason of a decrease in moving picture quality.
Further, in an inverse case where a luminance level in the transition region is low and the luminance level of the background is high, there is a phenomenon in which the luminance in the transition region is seen lighter than it should be, and the moving picture quality decreases by the same cause.
Japanese patent No. 3295437 (Patent Document 1) discloses a method of reducing indistinct edges without causing flicker. As shown in FIG. 39, this method generates an assumed (middle time point) virtual frame image and inserts the virtual frame image between the subsequent 2 frames. In this way, the indistinct edges are reduced, and degradation of moving picture quality is suppressed.    [Patent Document 1] Japanese Patent Publication Patent No. 3295437 publication (published on Jun. 24, 2002).